Guest Star: Brian Thomas Smith

Brian Thomas Smith is an American actor best known for his recurring role as Zach Johnson on The Big Bang Theory.

His TV credits include roles on shows including A Good Knight's Quest, Casual: The Series, and The Flip Side. He has had recurring role on TV shows such as The Indies, Wolfpack of Reseda, Suit Up, and The PET Squad Files. His TV credits also include guest appearances on Two and a Half Men and Happy Endings.

When Leonard goes to invite Penny to come see the guys' experiment on the roof, she is with a new guy, Zack, who she met at the Cheesecake Factory. After Zack asks some dumb questions, even Sheldon claims Penny could do better.

When the girls run into Penny's ex, Zack, during a girls' night out, Amy is overcome with new sensations which she struggles to understand. Sheldon helps Amy come to terms with her sexual feelings for Zack, encouraging her to follow her endocrine system.

After the guys hurt Zack's feelings, they try to make things up to him by taking him to the comic book store. The guys then decide that Zack would be the perfect addition to their Justice League group for the upcoming New Year's Eve costume party.

Penny and Leonard's Thanksgiving is thrown into disarray when Penny realizes that her "fake Vegas wedding" to her ex-boyfriend Zack is actually very real. Zack visits the guys as they spend Thanksgiving at Mrs. Wolowitz house so he can sign the annulment papers.

When Sheldon flies back to Texas for his sister's childbirth, the gang gather to decorate the apartment's Christmas tree. When they begin to wonder what life would be like without Sheldon, they come to realize how big a part of their lifes he has become.